In recent years, research and development have been extensively conducted on light-emitting elements utilizing electroluminescence. In a basic structure of such a light-emitting element, a light-emissive substance is interposed between a pair of electrodes. By voltage application to this element, light emission can be obtained from the light-emissive substance.
Since such a light-emitting element is of self-light-emitting type, it is considered that the light-emitting element has advantages over a liquid crystal display in that visibility of pixels is high, backlight is not required, and so on and is therefore suitable as flat panel display elements. Further, other advantages of such a light-emitting element are that the element can be manufactured to be thin and lightweight and the response speed is very high.
Since the light-emitting element can be formed into a film shape, light emission can be easily obtained from a flat surface with large-area. This is a feature which is difficult to be obtained by point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Accordingly, the light-emitting element is extremely effective for use as a flat light source applicable to illumination and the like.
Light-emitting elements utilizing electroluminescence are classified broadly according to whether they use an organic compound or an inorganic compound as a light-emissive substance.
When an organic compound is used as a light-emitting substance, electrons and holes are each injected into a layer including a light-emitting organic compound from a pair of electrodes by voltage application to a light-emitting element, so that a current flows therethrough. The electrons and holes (carriers) are recombined, and thus, the light-emitting organic compound is excited. The light-emitting organic compound relaxes to a ground state from the excited state, thereby emitting light. Based on this mechanism, such a light-emitting element is referred to as current excitation type light-emitting element.
Note that the excited state of an organic compound can be a singlet excited state or a triplet excited state, and luminescence from the singlet excited state is referred to as fluorescence, and luminescence from the triplet excited state is referred to as phosphorescence.
In improving element characteristics of such a light-emitting element, there are a number of problems which depend on a material used, and in order to solve the problems, improvement of an element structure, development of a material, and the like have been carried out.
For example, in Non-patent Document 1: Tetsuo TSUTSUI, and eight others, Japanese Journal of Applied Physics, Vol. 38, L1502-L1504 (1999), a hole-blocking layer is provided, whereby light is efficiently emitted from a light-emitting element using a phosphorescent material. However, as described in Non-patent Document 1, the hole-blocking layer does not have durability and a lifetime of the light-emitting element is extremely short. Thus, it has been desired to develop a light-emitting element of which light-emitting efficiency is high and lifetime is long.